Methods and systems for managing power generation and temperature control of an aerial vehicle operating in crosswind-flight mode

ABSTRACT

Methods and systems described herein relate to power generation control for an aerial vehicle of an air wind turbine (AWT). More specifically, the methods described herein relate to balancing power generation or preventing a component of the aerial vehicle from overheating using rotor speed control. An example method may include operating an aerial vehicle in a crosswind-flight mode to generate power. The aerial vehicle may include a rotor configured to help generate the power. While the aerial vehicle is in the crosswind-flight mode the method may include comparing a power output level of the aerial vehicle to a power threshold and, based on the comparison, adjusting operation of the rotor in a manner that generates an optimal amount of power or minimizes overheating of the aerial vehicle.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Power generation systems may convert chemical and/or mechanical energy(e.g., kinetic energy) to electrical energy for various applications,such as utility systems. As one example, a wind energy system mayconvert kinetic wind energy to electrical energy.

SUMMARY

Methods and systems for managing power generation of an aerial vehicleoperating in a crosswind-flight mode are described herein. Beneficially,such embodiments may help produce power output in an efficient manner asthe aerial vehicle operates in crosswind flight during variable windconditions (e.g., during wind speed increases and wind speed decreases).Further, embodiments described herein may help mitigate overheating ofpower generation components of the aerial vehicle by maintaining orreducing power generation as needed.

In one aspect, a method may comprise operating an aerial vehicle of anair wind turbine (AWT) in a crosswind-flight mode to generate power. Theaerial vehicle may be coupled to a ground station through a tether. Theaerial vehicle may include at least one rotor coupled to at least onegenerator for the purpose of power generation when the aerial vehicleoperates in the crosswind-flight mode. While the aerial vehicle is inthe crosswind-flight mode, the method may include determining a powergeneration state of the aerial vehicle. The power generation state maybe one of a plurality of power generation states of the aerial vehicle.The plurality of power generation states may include, but are notlimited to, an efficiency-limited power generation state and atemperature-limited power generation state. Other power generationstates are possible as well. The method may further include selecting,based on the determined power-generation state, a control scheme for oneor more power-generation components of the aerial vehicle. A firstcontrol scheme may be selected if the aerial vehicle is in theefficiency-limited power generation state. A second control scheme maybe selected if the aerial vehicle is in the temperature-limited powergeneration state. Additional or other control schemes may be selected aswell, and may be based on power generation states other than anefficiency-limited power generation state and a temperature-limitedpower generation state. The method may further include operating the oneor more power-generation components of the aerial vehicle according tothe selected control scheme.

In another aspect, an airborne wind turbine (AWT) system may comprise anaerial vehicle configured to operate in a crosswind-flight mode togenerate power, The aerial vehicle may be coupled to a ground stationthrough a tether. The aerial vehicle may include at least one rotorcoupled to at least one generator for the purpose of power generationwhen the aerial vehicle operates in the crosswind-flight mode. Thesystem may further include a control system configured to, while theaerial vehicle is in the crosswind-flight mode; determine a powergeneration state of the aerial vehicle. The power generation state maybe one of a plurality of power generation states of the aerial vehicle.The plurality of power generation states may include, but are notlimited to, an efficiency-limited power generation state and atemperature-limited power generation state. Other power generationstates are possible as well. The control system may be furtherconfigured to include selecting, based on the determinedpower-generation state, a control scheme for one or morepower-generation components of the aerial vehicle. A first controlscheme may be selected if the aerial vehicle is in theefficiency-limited power generation state. A second control scheme maybe selected if the aerial vehicle is in the temperature-limited powergeneration state. Additional or other control schemes may be selected aswell, and may be based on power generation states other than anefficiency-limited power generation state and a temperature-limitedpower generation state. The control system may also be furtherconfigured to operate the one or more power-generation components of theaerial vehicle according to the selected control scheme.

In another aspect, a method may comprise operating an aerial vehicle ofan air wind turbine (AWT) in a crosswind-flight mode to generate power.The aerial vehicle may be coupled to a ground station through a tether.The aerial vehicle may include at least one rotor coupled to at leastone generator for the purpose of power generation when the aerialvehicle operates in the crosswind-flight mode. While the aerial vehicleis in the crosswind-flight mode, the method may include determining apower generation state of the aerial vehicle. The power generation statemay be one of a plurality of power generation states of the aerialvehicle. The method may further include selecting, based on thedetermined power-generation state, a control scheme for one or morepower-generation components of the aerial vehicle and operating the oneor more power-generation components of the aerial vehicle according tothe selected control scheme.

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts an Airborne Wind Turbine (AWT), according to an exampleembodiment.

FIG. 2 is a simplified block diagram illustrating components of an AWT,according to an example embodiment.

FIGS. 3A and 3B depict an example of an aerial vehicle transitioningfrom hover flight to crosswind flight, according to an exampleembodiment.

FIG. 3C depicts an example of an aerial vehicle transitioning from hoverflight to crosswind flight in a tether sphere, according to an exampleembodiment.

FIG. 4 is a flowchart of a method, according to an example embodiment.

FIG. 5 illustrates a graphical representation of an operating scenario,according to an example embodiment.

FIG. 6 is a flowchart of a method, according to an example embodiment.

DETAILED DESCRIPTION

Exemplary methods and systems are described herein. It should beunderstood that the word “exemplary” is used herein to mean “serving asan example, instance, or illustration.” Any embodiment or featuredescribed herein as “exemplary” or “illustrative” is not necessarily tobe construed as preferred or advantageous over other embodiments orfeatures. More generally, the embodiments described herein are not meantto be limiting. It will be readily understood that certain aspects ofthe disclosed methods systems and can be arranged and combined in a widevariety of different configurations, all of which are contemplatedherein.

I. Overview

Illustrative embodiments relate to aerial vehicles, which may be used ina wind energy system, such as an Airborne Wind Turbine (AWT). Inparticular, illustrative embodiments may relate to or take the form ofmethods and systems for transitioning an aerial vehicle between certainflight modes that facilitate conversion of kinetic energy to electricalenergy.

By way of background, an AWT may include an aerial vehicle that flies ina path, such as a substantially circular path, to convert kinetic windenergy to electrical energy. In an illustrative implementation, theaerial vehicle may be connected to a ground station via a tether. Whiletethered, the aerial vehicle can: (i) fly at a range of elevations andsubstantially along the path, and return to the ground, and (ii)transmit electrical energy to the ground station via the tether. (Insome embodiments, the ground station may transmit electricity to theaerial vehicle for take-off and/or landing.)

In an AWT, an aerial vehicle may rest in and/or on a ground station (orperch) when the wind is not conducive to power generation. When the windis conducive to power generation, the ground station may deploy (orlaunch) the aerial vehicle. For example, in one embodiment, the aerialvehicle may be deployed when the wind speed is at or greater than 3.5meters per second (m/s) at an altitude of 200 meters (m), In addition,when the aerial vehicle is deployed and the wind is not conducive topower generation, the aerial vehicle may return to the ground station.

Moreover, in an AWT, an aerial vehicle may be configured for hoverflight and crosswind flight. Crosswind flight may be used to travel in amotion, such as a substantially circular motion, and thus may be theprimary technique that is used to generate electrical energy. Hoverflight in turn may be used by the aerial vehicle to prepare and positionitself for crosswind flight. In particular, the aerial vehicle couldascend to a location for crosswind flight based at least in part onhover flight. Further, the aerial vehicle could take-off and/or land viahover flight.

In hover flight, a span of a main wing of the aerial vehicle may beoriented substantially parallel to the ground, and one or morepropellers (or rotors) of the aerial vehicle may cause the aerialvehicle to hover over the ground. In some embodiments, the aerialvehicle may vertically ascend or descend in hover flight.

In crosswind flight, the aerial vehicle may be propelled by the windsubstantially along a path, which as noted above, may convert kineticwind energy to electrical energy. In some embodiments, the one or morepropellers of the aerial vehicle may generate electrical energy byslowing down the incident wind.

The aerial vehicle may enter crosswind flight when (i) the aerialvehicle has attached wind-flow (e.g., steady flow and/or no stallcondition (which may refer to no separation of air flow from anairfoil)) and (ii) the tether is under tension. Moreover, the aerialvehicle may enter crosswind flight at a location that is substantiallydownwind of the ground station.

In some embodiments, a tension of the tether during crosswind flight maybe greater than a tension of the tether during hover flight. Forinstance, in one embodiment, the tension of the tether during crosswindflight may be 15 kilonewtons (KN), and the tension of the tether duringhover flight may be 1 KN.

In line with the discussion above, the aerial vehicle may generateelectrical energy in crosswind flight and may thereby allow the AWT toextract useful power from the wind. The aerial vehicle may generateelectrical energy in various wind conditions such as high wind speeds,large gusts, turbulent air, or variable wind conditions. Generally, theinertial speed of the aerial vehicle, the tension of the tether, and thepower output of the AWT increase as the wind speed increases.Additionally, the power output typically has a maximum effective limit(rated power output). The wind speed at which the power output limit isreached is defined as the rated wind speed. Additionally, the powergeneration components of the AWT may produce heat, and as power outputincreases, the heat production may increase, potentially limiting theoperational parameters of the AWT. Therefore, it may be desirable toenact control schemes that control the power generation components andtherefore control their heat production.

Considering this, disclosed embodiments may allow for operating anaerial vehicle of an AWT in crosswind-flight in a manner that mayefficiently generate power generation in variable wind conditions suchas those noted above and/or may control and/or limit the heat producedby power generation components in the aerial vehicle.

II. Illustrative Systems

A. Airborne Wind Turbine (AWT)

FIG. 1 depicts an AWT 100, according to an example embodiment. Inparticular, the AWT 100 includes a ground station 110, a tether 120, andan aerial vehicle 130. As shown in FIG. 1, the aerial vehicle 130 may beconnected to the tether 120, and the tether 120 may be connected to theground station 110. In this example, the tether 120 may be attached tothe ground station 110 at one location on the ground station 110, andattached to the aerial vehicle 130 at two locations on the aerialvehicle 130. However, in other examples, the tether 120 may be attachedat multiple locations to any part of the ground station 110 and/or theaerial vehicle 130.

The ground station 110 may be used to hold and/or support the aerialvehicle 130 until it is in an operational mode. The ground station 110may also be configured to allow for the repositioning of the aerialvehicle 130 such that deploying of the device is possible. Further, theground station 110 may be further configured to receive the aerialvehicle 130 during a landing. The ground station 110 may be formed ofany material that can suitably keep the aerial vehicle 130 attachedand/or anchored to the ground while in hover flight, forward flight,crosswind flight. In some implementations, a ground station 110 may beconfigured for use on land. However, a ground station 110 may also beimplemented on a body of water, such as a lake, river, sea, or ocean.For example, a ground station could include or be arranged on a floatingoff-shore platform or a boat, among other possibilities. Further, aground station 110 may be configured to remain stationary or to moverelative to the ground or the surface of a body of water.

In addition, the ground station 110 may include one or more components(not shown), such as a winch, that may vary a length of the tether 120.For example, when the aerial vehicle 130 is deployed, the one or morecomponents may be configured to pay out and/or reel out the tether 120.In some implementations, the one or more components may be configured topay out and/or reel out the tether 120 to a predetermined length. Asexamples, the predetermined length could be equal to or less than amaximum length of the tether 120. Further, when the aerial vehicle 130lands in the ground station 110, the one or more components may beconfigured to reel in the tether 120.

The tether 120 may transmit electrical energy generated by the aerialvehicle 130 to the ground station 110. In addition, the tether 120 maytransmit electricity to the aerial vehicle 130 in order to power theaerial vehicle 130 for takeoff, landing, hover flight, and/or forwardflight. The tether 120 may be constructed in any form and using anymaterial which may allow for the transmission, delivery, and/orharnessing of electrical energy generated by the aerial vehicle 130and/or transmission of electricity to the aerial vehicle 130. The tether120 may also be configured to withstand one or more forces of the aerialvehicle 130 when the aerial vehicle 130 is in an operational mode. Forexample, the tether 120 may include a core configured to withstand oneor more forces of the aerial vehicle 130 when the aerial vehicle 130 isin hover flight, forward flight, and/or crosswind flight. The core maybe constructed of any high strength fibers. In some examples, the tether120 may have a fixed length and/or a variable length. For instance, inat least one such example, the tether 120 may have a length of 140meters.

The aerial vehicle 130 may be configured to fly substantially along apath 150 to generate electrical energy. The term “substantially along,”as used in this disclosure, refers to exactly along and/or one or moredeviations from exactly along that do not significantly impactgeneration of electrical energy as described herein and/or transitioningan aerial vehicle between certain flight modes as described herein.

The aerial vehicle 130 may include or take the form of various types ofdevices, such as a kite, a helicopter, a wing and/or an airplane, amongother possibilities. The aerial vehicle 130 may be formed of solidstructures of metal, plastic and/or other polymers. The aerial vehicle130 may be formed of any material which allows for a highthrust-to-weight ratio and generation of electrical energy which may beused in utility applications. Additionally, the materials may be chosento allow for a lightning hardened, redundant and/or fault tolerantdesign which may be capable of handling large and/or sudden shifts inwind speed and wind direction. Other materials may be possible as well.

The path 150 may be various different shapes in various differentembodiments. For example, the path 150 may be substantially circular.And in at least one such example, the path 150 may have a radius of upto 265 meters. The term “substantially circular,” as used in thisdisclosure, refers to exactly circular and/or one or more deviationsfrom exactly circular that do not significantly impact generation ofelectrical energy as described herein. Other shapes for the path 150 maybe an oval, such as an ellipse, the shape of a jelly bean, the shape ofthe number of 8, etc.

As shown in FIG. 1, the aerial vehicle 130 may include a main wing 131,a front section 132, rotor connectors 133A-B, rotors 134A-D, a tail boom135, a tail wing 136, and a vertical stabilizer 137. Any of thesecomponents may be shaped in any form which allows for the use ofcomponents of lift to resist gravity and/or move the aerial vehicle 130forward.

The main wing 131 may provide a primary lift for the aerial vehicle 130.The main wing 131 may be one or more rigid or flexible airfoils, and mayinclude various control surfaces, such as winglets, flaps, rudders,elevators, etc. The control surfaces may be used to stabilize the aerialvehicle 130 and/or reduce drag on the aerial vehicle 130 during hoverflight, forward flight, and/or crosswind flight.

The main wing 131 may be any suitable material for the aerial vehicle130 to engage in hover flight, forward flight, and/or crosswind flight.For example, the main wing 131 may include carbon fiber and/or e-glass.Moreover, the main wing 131 may have a variety dimensions. For example,the main wing 131 may have one or more dimensions that correspond with aconventional wind turbine blade. As another example, the main wing 131may have a span of 8 meters, an area of 4 meters squared, and an aspectratio of 15. The front section 132 may include one or more components,such as a nose, to reduce drag on the aerial vehicle 130 during flight.

The rotor connectors 133A-B may connect the rotors 134A-D to the mainwing 131. In some examples, the rotor connectors 133A-B may take theform of or be similar in form to one or more pylons. In this example,the rotor connectors 133A-B are arranged such that the rotors 134A-D arespaced between the main wing 131. In some examples, a vertical spacingbetween corresponding rotors (e.g., rotor 134A and rotor 134B or rotor134C and rotor 134D) may be 0.9 meters.

The rotors 134A-D may be configured to drive one or more generators forthe purpose of generating electrical energy. In this example, the rotors134A-D may each include one or more blades, such as three blades. Theone or more rotor blades may rotate via interactions with the wind andwhich could be used to drive the one or more generators. In addition,the rotors 134A-D may also be configured to provide a thrust to theaerial vehicle 130 during flight. With this arrangement, the rotors134A-D may function as one or more propulsion units, such as apropeller. Although the rotors 134A-D are depicted as four rotors inthis example, in other examples the aerial vehicle 130 may include anynumber of rotors, such as less than four rotors or more than fourrotors.

The tail boom 135 may connect the main wing 131 to the tail wing 136.The tail boom 135 may have a variety of dimensions. For example, thetail boom 135 may have a length of 2 meters. Moreover, in someimplementations, the tail boom 135 could take the form of a body and/orfuselage of the aerial vehicle 130. And in such implementations, thetail boom 135 may carry a payload.

The tail wing 136 and/or the vertical stabilizer 137 may be used tostabilize the aerial vehicle and/or reduce drag on the aerial vehicle130 during hover flight, forward flight, and/or crosswind flight. Forexample, the tail wing 136 and/or the vertical stabilizer 137 may beused to maintain a pitch of the aerial vehicle 130 during hover flight,forward flight, and/or crosswind flight. In this example, the verticalstabilizer 137 is attached to the tail boom 135, and the tail wing 136is located on top of the vertical stabilizer 137. The tail wing 136 mayhave a variety of dimensions. For example, the tail wing 136 may have alength of 2 meters. Moreover, in some examples, the tail wing 136 mayhave a surface area of 0.45 meters squared. Further, in some examples,the tail wing 136 may be located 1 meter above a center of mass of theaerial vehicle 130.

While the aerial vehicle 130 has been described above, it should beunderstood that the methods and systems described herein could involveany suitable aerial vehicle that is connected to a tether, such as thetether 120.

B. Illustrative Components of an AWT

FIG. 2 is a simplified block diagram illustrating components of the AWT200. The AWT 200 may take the form of or be similar in form to the AWT100. In particular, the AWT 200 includes a ground station 210, a tether220, and an aerial vehicle 230. The ground station 210 may take the formof or be similar in form to the ground station 110, the tether 220 maytake the form of or be similar in form to the tether 120, and the aerialvehicle 230 may take the form of or be similar in form to the aerialvehicle 130.

As shown in FIG. 2, the ground station 210 may include one or moreprocessors 212, data storage 214, and program instructions 216. Aprocessor 212 may be a general-purpose processor or a special purposeprocessor (e.g., digital signal processors, application specificintegrated circuits, etc.). The one or more processors 212 can beconfigured to execute computer-readable program instructions 216 thatare stored in a data storage 214 and are executable to provide at leastpart of the functionality described herein.

The data storage 214 may include or take the form of one or morecomputer-readable storage media that may be read or accessed by at leastone processor 212. The one or more computer-readable storage media caninclude volatile and/or non-volatile storage components, such asoptical, magnetic, organic or other memory or disc storage, which may beintegrated in whole or in part with at least one of the one or moreprocessors 212. In some embodiments, the data storage 214 may beimplemented using a single physical device (e.g., one optical, magnetic,organic or other memory or disc storage unit), while in otherembodiments, the data storage 214 can be implemented using two or morephysical devices.

As noted, the data storage 214 may include computer-readable programinstructions 216 and perhaps additional data, such as diagnostic data ofthe ground station 210. As such, the data storage 214 may includeprogram instructions to perform or facilitate some or all of thefunctionality described herein.

In a further respect, the ground station 210 may include a communicationsystem 218. The communications system 218 may include one or morewireless interfaces and/or one or more wireline interfaces, which allowthe ground station 210 to communicate via one or more networks. Suchwireless interfaces may provide for communication under one or morewireless communication protocols, such as Bluetooth, WiFi (e.g., an IEEE802.11 protocol), Long-Term Evolution (LTE), WiMAX (e.g., an IEEE 802.16standard), a radio-frequency ID (RFID) protocol, near-fieldcommunication (NFC), and/or other wireless communication protocols. Suchwireline interfaces may include an Ethernet interface, a UniversalSerial Bus (USB) interface, or similar interface to communicate via awire, a twisted pair of wires, a coaxial cable, an optical link, afiber-optic link, or other physical connection to a wireline network.The ground station 210 may communicate with the aerial vehicle 230,other ground stations, and/or other entities (e.g., a command center)via the communication system 218.

In an example embodiment, the ground station 210 may includecommunication systems 218 that allows for both short-range communicationand long-range communication. For example, the ground station 210 may beconfigured for short-range communications using Bluetooth and forlong-range communications under a CDMA protocol. In such an embodiment,the ground station 210 may be configured to function as a “hot spot”; orin other words, as a gateway or proxy between a remote support device(e.g., the tether 220, the aerial vehicle 230, and other groundstations) and one or more data networks, such as cellular network and/orthe Internet. Configured as such, the ground station 210 may facilitatedata communications that the remote support device would otherwise beunable to perform by itself.

For example, the ground station 210 may provide a WiFi connection to theremote device, and serve as a proxy or gateway to a cellular serviceprovider's data network, which the ground station 210 might connect tounder an LTE or a 3G protocol, for instance. The ground station 210could also serve as a proxy or gateway to other ground stations or acommand station, which the remote device might not be able to otherwiseaccess.

Moreover, as shown in FIG. 2, the tether 220 may include transmissioncomponents 222 and a communication link 224. The transmission components222 may be configured to transmit electrical energy from the aerialvehicle 230 to the ground station 210 and/or transmit electrical energyfrom the ground station 210 to the aerial vehicle 230. The transmissioncomponents 222 may take various different forms in various differentembodiments. For example, the transmission components 222 may includeone or more conductors that are configured to transmit electricity. Andin at least one such example, the one or more conductors may includealuminum and/or any other material which allows for the conduction ofelectric current. Moreover, in some implementations, the transmissioncomponents 222 may surround a core of the tether 220 (not shown).

The ground station 210 could communicate with the aerial vehicle 230 viathe communication link 224. The communication link 224 may bebidirectional and may include one or more wired and/or wirelessinterfaces. Also, there could be one or more routers, switches, and/orother devices or networks making up at least a part of the communicationlink 224.

Further, as shown in FIG. 2, the aerial vehicle 230 may include one ormore sensors 232, a power system 234, power generation/conversioncomponents 236, a communication system 238, one or more processors 242,data storage 244, and program instructions 246, and a control system248.

The sensors 232 could include various different sensors in variousdifferent embodiments. For example, the sensors 232 may include a globala global positioning system (GPS) receiver. The GPS receiver may beconfigured to provide data that is typical of well-known GPS systems(which may be referred to as a global navigation satellite system(GNNS)), such as the GPS coordinates of the aerial vehicle 230. Such GPSdata may be utilized by the AWT 200 to provide various functionsdescribed herein.

As another example, the sensors 232 may include one or more windsensors, such as one or more pitot tubes. The one or more wind sensorsmay be configured to detect apparent and/or relative wind. Such winddata may be utilized by the AWT 200 to provide various functionsdescribed herein.

Still as another example, the sensors 232 may include an inertialmeasurement unit (IMU). The IMU may include both an accelerometer and agyroscope, which may be used together to determine the orientation ofthe aerial vehicle 230. In particular, the accelerometer can measure theorientation of the aerial vehicle 230 with respect to earth, while thegyroscope measures the rate of rotation around an axis, such as acenterline of the aerial vehicle 230. IMUs are commercially available inlow-cost, low-power packages. For instance, the IMU may take the form ofor include a miniaturized MicroElectroMechanical System (MEMS) or aNanoElectroMechanical System (NEMS). Other types of IMUs may also beutilized. The IMU may include other sensors, in addition toaccelerometers and gyroscopes, which may help to better determineposition. Two examples of such sensors are magnetometers and pressuresensors. Other examples of sensors are also possible.

While an accelerometer and gyroscope may be effective at determining theorientation of the aerial vehicle 230, slight errors in measurement maycompound over time and result in a more significant error. However, anexample aerial vehicle 230 may be able mitigate or reduce such errors byusing a magnetometer to measure direction. One example of a magnetometeris a low-power, digital 3-axis magnetometer, which may be used torealize an orientation independent electronic compass for accurateheading information. However, other types of magnetometers may beutilized as well.

The aerial vehicle 230 may also include a pressure sensor or barometer,which can be used to determine the altitude of the aerial vehicle 230.Alternatively, other sensors, such as sonic altimeters or radaraltimeters, can be used to provide an indication of altitude, which mayhelp to improve the accuracy of and/or prevent drift of the IMU.

As noted, the aerial vehicle 230 may include the power system 234. Thepower system 234 could take various different forms in various differentembodiments. For example, the power system 234 may include one or morebatteries for providing power to the aerial vehicle 230. In someimplementations, the one or more batteries may be rechargeable and eachbattery may be recharged via a wired connection between the battery anda power supply and/or via a wireless charging system, such as aninductive charging system that applies an external time-varying magneticfield to an internal battery and/or charging system that uses energycollected from one or more solar panels.

As another example, the power system 234 may include one or more motorsor engines for providing power to the aerial vehicle 230. In someimplementations, the one or more motors or engines may be powered by afuel, such as a hydrocarbon-based fuel. And in such implementations, thefuel could be stored on the aerial vehicle 230 and delivered to the oneor more motors or engines via one or more fluid conduits, such aspiping. In some implementations, the power system 234 may be implementedin whole or in part on the ground station 210.

As noted, the aerial vehicle 230 may include the powergeneration/conversion components 236. The power generation/conversioncomponents 326 could take various different forms in various differentembodiments. For example, the power generation/conversion components 236may include one or more generators, such as high-speed, direct-drivegenerators. With this arrangement, the one or more generators may bedriven by one or more rotors, such as the rotors 134A-D. And in at leastone such example, the one or more generators may operate at full ratedpower wind speeds of 11.5 meters per second at a capacity factor whichmay exceed 60 percent, and the one or more generators may generateelectrical power from 40 kilowatts to 600 megawatts.

Moreover, as noted, the aerial vehicle 230 may include a communicationsystem 238. The communication system 238 may take the form of or besimilar in form to the communication system 218. The aerial vehicle 230may communicate with the ground station 210, other aerial vehicles,and/or other entities (e.g., a command center) via the communicationsystem 238.

In some implementations, the aerial vehicle 230 may be configured tofunction as a “hot spot”; or in other words, as a gateway or proxybetween a remote support device (e.g., the ground station 210, thetether 220, other aerial vehicles) and one or more data networks, suchas cellular network and/or the Internet. Configured as such, the aerialvehicle 230 may facilitate data communications that the remote supportdevice would otherwise be unable to perform by itself.

For example, the aerial vehicle 230 may provide a WiFi connection to theremote device, and serve as a proxy or gateway to a cellular serviceprovider's data network, which the aerial vehicle 230 might connect tounder an LTE or a 3G protocol, for instance. The aerial vehicle 230could also serve as a proxy or gateway to other aerial vehicles or acommand station, which the remote device might not be able to otherwiseaccess.

As noted, the aerial vehicle 230 may include the one or more processors242, the program instructions 244, and the data storage 246. The one ormore processors 242 can be configured to execute computer-readableprogram instructions 246 that are stored in the data storage 244 and areexecutable to provide at least part of the functionality describedherein. The one or more processors 242 may take the form of or besimilar in form to the one or more processors 212, the data storage 244may take the form of or be similar in form to the data storage 214, andthe program instructions 246 may take the form of or be similar in formto the program instructions 216.

Moreover, as noted, the aerial vehicle 230 may include the controlsystem 248. In some implementations, the control system 248 may beconfigured to perform one or more functions described herein. Thecontrol system 248 may be implemented with mechanical systems and/orwith hardware, firmware, and/or software. As one example, the controlsystem 248 may take the form of program instructions stored on anon-transitory computer readable medium and a processor that executesthe instructions. The control system 248 may be implemented in whole orin part on the aerial vehicle 230 and/or at least one entity remotelylocated from the aerial vehicle 230, such as the ground station 210.Generally, the manner in which the control system 248 is implemented mayvary, depending upon the particular application.

While the aerial vehicle 230 has been described above, it should beunderstood that the methods and systems described herein could involveany suitable aerial vehicle that is connected to a tether, such as thetether 230 and/or the tether 110.

C. Transitioning an Aerial Vehicle from Hover Flight to Crosswind Flightto Generate Power

FIGS. 3A and 3B depict an example 300 of transitioning an aerial vehiclefrom hover flight to crosswind flight in a manner such that power may begenerated, according to an example embodiment. Example 300 is generallydescribed by way of example as being carried out by the aerial vehicle130 described above in connection with FIG. 1. For illustrativepurposes, example 300 is described in a series of actions as shown inFIGS. 3A and 3B, though example 300 could be carried out in any numberof actions and/or combination of actions.

As shown in FIG. 3A, the aerial vehicle 130 may be connected to thetether 120, and the tether 120 is connected to the ground station 110.The ground station 110 is located on ground 302. Moreover, as shown inFIG. 3A, the tether 120 defines a tether sphere 304 having a radiusbased on a length of the tether 120, such as a length of the tether 120when it is extended. Example 300 may be carried out in and/orsubstantially on a portion 304A of the tether sphere 304. The term“substantially on,” as used in this disclosure, refers to exactly onand/or one or more deviations from exactly on that do not significantlyimpact transitioning an aerial vehicle between certain flight modes asdescribed herein.

Example 300 begins at a point 306 with deploying the aerial vehicle 130from the ground station 110 in a hover-flight orientation. With thisarrangement, the tether 120 may be paid out and/or reeled out. In someimplementations, the aerial vehicle 130 may be deployed when wind speedsincrease above a threshold speed (e.g., 3.5 m/s) at a threshold altitude(e.g., over 200 meters above the ground 302).

Further, at point 306 the aerial vehicle 130 may be operated in thehover-flight orientation. When the aerial vehicle 130 is in thehover-flight orientation, the aerial vehicle 130 may engage in hoverflight. For instance, when the aerial vehicle engages in hover flight,the aerial vehicle 130 may ascend, descend, and/or hover over the ground302. When the aerial vehicle 130 is in the hover-flight orientation, aspan of the main wing 131 of the aerial vehicle 130 may be orientedsubstantially perpendicular to the ground 302. The term “substantiallyperpendicular,” as used in this disclosure, refers to exactlyperpendicular and/or one or more deviations from exactly perpendicularthat do not significantly impact transitioning an aerial vehicle betweencertain flight modes as described herein.

Example 300 continues at a point 308 with while the aerial vehicle 130is in the hover-flight orientation positioning the aerial vehicle 130 ata first location 310 that is substantially on the tether sphere 304. Asshown in FIG. 3A, the first location 310 may be in the air andsubstantially downwind of the ground station 110.

The term “substantially downwind,” as used in this disclosure, refers toexactly downwind and/or one or more deviations from exactly downwindthat do not significantly impact transitioning an aerial vehicle betweencertain flight modes as described herein.

For example, the first location 310 may be at a first angle from an axisextending from the ground station 110 that is substantially parallel tothe ground 302. In some implementations, the first angle may be 30degrees from the axis. In some situations, the first angle may bereferred to as azimuth, and the first angle may be between 30 degreesclockwise from the axis and 330 degrees clockwise from the axis, such as15 degrees clockwise from the axis or 345 degrees clockwise from theaxis.

As another example, the first location 310 may be at a second angle fromthe axis. In some implementations, the second angle may be 10 degreesfrom the axis. In some situations, the second angle may be referred toas elevation, and the second angle may be between 10 degrees in adirection above the axis and 10 degrees in a direction below the axis.The term “substantially parallel,” as used in this disclosure refers toexactly parallel and/or one or more deviations from exactly parallelthat do not significantly impact transitioning an aerial vehicle betweencertain flight modes described herein.

At point 308, the aerial vehicle 130 may accelerate in the hover-flightorientation. For example, at point 308, the aerial vehicle 130 mayaccelerate up to a few meters per second. In addition, at point 308, thetether 120 may take various different forms in various differentembodiments. For example, as shown in FIG. 3A, at point 308 the tether120 may be extended. With this arrangement, the tether 120 may be in acatenary configuration. Moreover, at point 306 and point 308, a bottomof the tether 120 may be a predetermined altitude 312 above the ground302. With this arrangement, at point 306 and point 308 the tether 120may not contact the ground 302.

Example 300 continues at point 314 with transitioning the aerial vehicle130 from the hover-flight orientation to a forward-flight orientation,such that the aerial vehicle 130 moves from the tether sphere 304. Asshown in FIG. 3B, the aerial vehicle 130 may move from the tether sphere304 to a location toward the ground station 110 (which may be referredto as being inside the tether sphere 304).

When the aerial vehicle 130 is in the forward-flight orientation, theaerial vehicle 130 may engage in forward flight (which may be referredto as airplane-like flight). For instance, when the aerial vehicle 130engages in forward flight, the aerial vehicle 130 may ascend. Theforward-flight orientation of the aerial vehicle 130 could take the formof an orientation of a fixed-wing aircraft (e.g., an airplane) inhorizontal flight. In some examples, transitioning the aerial vehicle130 from the hover-flight orientation to the forward-flight orientationmay involve a flight maneuver, such as pitching forward. And in such anexample, the flight maneuver may be executed within a time period, suchas less than one second.

At point 314, the aerial vehicle 130 may achieve attached flow. Further,at point 314, a tension of the tether 120 may be reduced. With thisarrangement, a curvature of the tether 120 at point 314 may be greaterthan a curvature of the tether 120 at point 308. As one example, atpoint 314, the tension of the tether 120 may be less than 1 KN, such as500 newtons (N).

Example 300 continues at one or more points 318 with operating theaerial vehicle 130 in the forward-flight orientation to ascend at anangle of ascent to a second location 320 that is substantially on thetether sphere 304. As shown in FIG. 3B, the aerial vehicle 130 may flysubstantially along a path 316 during the ascent at one or more points318. In this example, one or more points 318 is shown as three points, apoint 318A, a point 318B, and a point 318C. However, in other examples,one or more points 318 may include less than three or more than threepoints.

In some examples, the angle of ascent may be an angle between the path316 and the ground 302. Further, the path 316 may take various differentforms in various different embodiments. For instance, the path 316 maybe a line segment, such as a chord of the tether sphere 304.

As shown in FIG. 3B, the second location 320 may be in the air andsubstantially downwind of the ground station 110. The second location320 may be oriented with respect to the ground station 110 the similarway as the first location 310 may be oriented with respect to the groundstation 110.

For example, the second location 320 may be at a first angle from anaxis extending from the ground station 110 that is substantiallyparallel to the ground 302. In some implementations, the first angle maybe 30 degrees from the axis. In some situations, the first angle may bereferred to as azimuth, and the angle may be between 30 degreesclockwise from the axis and 330 degrees clockwise from the axis, such as15 degrees clockwise from the axis or 345 degrees clockwise from theaxis.

In addition, as shown in FIG. 3B, the second location 320 may besubstantially upwind of the first location 310. The term “substantiallyupwind,” as used in this disclosure, refers to exactly upwind and/or oneor more deviations from exactly upwind that do not significantly impacttransitioning an aerial vehicle between certain flight modes asdescribed herein.

At one or more points 318, a tension of the tether 120 may increaseduring the ascent. For example, a tension of the tether 120 at point318C may be greater than a tension of the tether 120 at point 318B, atension of the tether 120 at point 318B may be greater than a tension ofthe tether 120 at point 318A. Further, a tension of the tether 120 atpoint 318A may be greater than a tension of the tether at point 314.

With this arrangement, a curvature of the tether 120 may decrease duringthe ascent. For example, a curvature the tether 120 at point 318C may beless than a curvature the tether at point 318B, and a curvature of thetether 120 at point 318B may be less than a curvature of the tether atpoint 318A. Further, in some examples, a curvature of the tether 120 atpoint 318A may be less than a curvature of the tether 120 at point 314.

Example 300 continues at a point 322 with transitioning the aerialvehicle 130 from the forward-flight orientation to a crosswind-flightmode. In some examples, transitioning the aerial vehicle 130 from theforward-flight orientation to the crosswind-flight mode may involve aflight maneuver. When the aerial vehicle 130 is in the crosswind-flightmode, the aerial vehicle 130 may engage in crosswind flight. Forinstance, when the aerial vehicle 130 engages in crosswind flight, theaerial vehicle 130 may fly, at a multiple of attached-wind flow (notshown in FIG. 3B) substantially along a path, such as path 150, togenerate electrical energy. In some implementations, a natural rolland/or yaw of the aerial vehicle 130 may occur during crosswind flight.

FIG. 3C depicts example 300 from a three-dimensional (3D) perspective.Accordingly, like numerals may denote like entities. As noted above,tether sphere 304 has a radius based on a length of a tether 120, suchas a length of the tether 120 when it is extended. Also as noted above,in FIG. 3C, the tether 120 is connected to ground station 310, and theground station 310 is located on ground 302. Further, relative wind 303contacts the tether sphere 304. Note, in FIG. 3C, only a portion of thetether sphere 304 that is above the ground 302 is depicted. The portionmay be described as one half of the tether sphere 304.

As shown in FIG. 3C, the first portion 304A of the tether sphere 304 issubstantially downwind of the ground station 310. In FIG. 3C, the firstportion 304A may be described as one quarter of the tether sphere 304.

Like FIG. 3B, FIG. 3C depicts transitioning aerial vehicle 130 (notshown in FIG. 3C to simply the Figure) between hover flight andcrosswind flight. As shown in FIG. 3C, when the aerial vehicle 130transitions from the hover-flight orientation to a forward-flightorientation, the aerial vehicle may be positioned at a point 314 that isinside the first portion 304A of the tether sphere 304. Further still,as shown in FIG. 3C, when aerial vehicle 130 ascends in theforward-flight orientation to a location 320 that is substantially onthe first portion 304A of the tether sphere 304, the aerial vehicle mayfollow a path 316. Yet even further, as shown in FIG. 3C, aerial vehicle130 may then transition from location 320 in a forward-flightorientation to a crosswind-flight mode at location 322, for example.

III. Ilustrative Methods

FIG. 4 is a flowchart illustrating a method 400, according to an exampleembodiment. The method 400 may be used to control rotor operation of anaerial vehicle of an AWT. More specifically, the method 400 may be usedto control one or more rotors of an aerial vehicle while the aerialvehicle is in a crosswind-flight mode in a manner that may control powergeneration and/or prevent or limit overheating of components of theaerial vehicle. Illustrative methods, such as method 400, may be carriedout in whole or in part by a component or components of an AWT, such asby the one or more components of the AWT 100 shown in FIG. 1 and the AWT200 shown in FIG. 2. For simplicity, method 400 may be describedgenerally as being carried out by an aerial vehicle of an AWT, such asthe aerial vehicle 130 of AWT 100 and/or the aerial vehicle 230 of AWT200. However, it should be understood that example methods, such asmethod 400, may be carried out by other entities or combinations ofentities without departing from the scope of the disclosure.

As shown by block 402, method 400 involves operating an aerial vehicleof an AWT in a crosswind-flight mode to generate power. The aerialvehicle may, for example, operate along a particular flight path andwhile operating along the flight path, the aerial vehicle may operateone or more rotors similar to or the same as rotors 134A-D to generatethe power. The flight path may be constrained by a tether such as tether120 and, as noted above, the tether may define a tether sphere having aradius based on a length of the tether. For example, the tether spheremay be the same as or similar to tether sphere 304 of FIGS. 3A-3C. Theflight path may be substantially on the tether sphere and may include asubstantially circular path (e.g., path 150) that allows the aerialvehicle to generate the power.

Within this disclosure, the term “substantially circular” refers toexactly circular and/or one or more deviations from exactly circularthat does not significantly impact the aerial vehicle from generatingpower. Substantially circular paths may include, for example,oval-shaped paths, balloon-shaped paths, and bowl-shaped paths to name afew. Other substantially circular paths are possible as well.

To begin operating along the flight path, the aerial vehicle may bedeployed, may engage in hover flight, may engage in forward flight, andmay then transition to the first flight path on the tether sphere. Forexample, at block 402, the aerial vehicle may be operated in the same ora similar way as the aerial vehicle 130 may be operated whentransitioning from a hover flight orientation to a crosswind flightorientation as described with reference to example 300 of FIGS. 3A-3C.Accordingly, when operating along the first flight path in thecrosswind-flight mode, the aerial vehicle may be oriented the same as orsimilar to aerial vehicle 130 at point 322 of FIGS. 3B and 3C.

Note, in other examples, some of the above referenced flight maneuversmay be omitted. For instance, in some examples, the aerial vehicle maybe deployed, engage in forward flight to a position on the tethersphere, and thereafter immediately transition to the first flight path.Thus, in such examples, the aerial vehicle may omit the hover flightmaneuver.

To generate power, as noted above, while the aerial vehicle operatesalong the flight path, the one or more rotors may be configured to driveone or more generators for the purpose of generating electrical energy.The one or more rotors may each include one or more blades (e.g., threeblades) that may rotate via interactions with the wind and which couldbe used to drive the one or more generators. In practice, the blades ofthe rotors may act as barriers to the wind and when the wind forces theblades to move, the wind may transfer some of its energy to the rotorsvia the rotation of the blades. As the rotor rotates, it may drive thegenerator and the generator may generate power. The power generated maybe directly proportional to the rotational speed of the rotors.Accordingly, the faster the wind is applied to the blades of the rotors,the more electrical energy may be generated and eventually captured bythe AWT.

At block 404, while the aerial vehicle is in crosswind-flight mode,method 400 includes determining a power generation state. For reference,FIG. 5 helps illustrates exemplary power generation states.

FIG. 5 illustrates a graphical representation of an operating scenario,according to an example embodiment. The vertical axis represents powergeneration of the aerial vehicle, and the horizontal axis representswind speed. The aerial vehicle may produce power according to a powercurve 503. The power curve 503 may represent the amount of power theaerial vehicle may generate as a function of wind speed. In oneembodiment, the aerial vehicle may begin generating power when windspeeds are above the minimum level indicated by line 502 (e.g., 3.5meters-per-second). Note, within the context of this disclosure, thepower generation curve 503, wind speeds, and power generation levelsused in FIG. 5 are not intended to be limiting and other wind speeds andpower generation levels may be possible.

FIG. 5 also illustrates some exemplary power generation states. SegmentA illustrates a section of power curve 503 where the aerial vehicle isgenerating power (i.e., it is operating above zero threshold level 502a), but producing less than the rated power 506 a, and less than thethreshold level 504 a, where the power generation components may beginto be limited by rising temperatures. In this segment of the power curve503, the AWT may attempt to capture as much power as possible in themost efficient manner possible. Stated differently, while operatingalong segment A, the AWT may attempt to generate the maximum poweravailable from the wind. This may be referred to as anefficiency-limited power generation state.

At wind speeds greater than those indicated by line 504, the heatproduced by the power generation components may limit the ability ofthose components to generate power. At the threshold level indicated by504 a, the incremental increase in power generation per unit of windspeed drops due to the effect of heating in the power generationcomponents. This is illustrated by the changing slope in the power curve503 when it crosses the threshold level 504 a. At this point, the aerialvehicle may still be operating at less than the rated power 506 a.Accordingly, segment “B” illustrates a portion of power curve 503 wherethe aerial vehicle is generating power less than the rated power 506 a,and where the power generation components of the aerial vehicle arelimited by temperature concerns. This may be referred to as atemperature-limited power generation state. In this section of powercurve 503, it may be desirable to operate the aerial vehicle in a mannerthat controls the heat production of the power generation components.

At wind speeds greater than those indicated by line 506, the aerialvehicle may be operating at its maximum rated power. Segment Cillustrates a portion of power curve 503 where the aerial vehicle isproducing power at the rated power threshold level 506 a. This may bereferred to as a power-limited power generation state. In this sectionof power curve 503, it may be desirable to operate the aerial vehicle ina manner that controls both the power generation heat production of thepower generation components.

Referring again to FIG. 4, at block 404 the method involves determininga power generation state. One method of determining the power generationstate is to determine the amount of generated power and to evaluate thegenerated power amount in relation to known power threshold levels, suchas threshold levels 502, 504, and 506 described in relation to FIG. 5.Thus, the AWT could determine whether it is operating in, for example,an efficiency-limited power generation state, a temperature-limitedpower generation state, or a power-limited power generation state.

To measure the power generation amount, the AWT may use, for example, apower sensing element of sensors 232 that may continuously sense a poweroutput of the aerial vehicle. Upon determining the power generationamount, the comparison to one or more power threshold levels may bemade, for example, using a control system similar to or the same ascontrol system 248 and one or more processors similar to or the same asprocessors 242 and/or processors 212. Based on the comparison, it may bedetermined that the AWT is producing power at a power output level thatis equal to or less than the power threshold and/or a power output levelthat is equal to or greater than the power threshold.

Another method of determining the power generation state is to determinethe wind speed in which the aerial vehicle is operating and, bycomparing wind speed to a known power generation curve for the AWT, theamount of power generation. The power generation amount could then becompared to threshold levels as previously described.

To measure the wind speed, the aerial vehicle may use, for example, oneor more pitot tubes corresponding to sensors 232, along with processors242, and control system 248. For instance, the aerial vehicle may usecontrol system 248 to cause a pitot tube to be positioned directly intothe wind. In some examples, the aerial vehicle may use the pitot tube toobtain a large number of successive measurements of the wind or periodicmeasurements of the wind when measuring the wind speed. Successivemeasurements may be multiple measurements made using the pitot tubeoccurring over time and may be continuous or may occur in intervals. Inother examples, multiple pitot tubes may be used to measure wind speed.In other examples, wind speed may be measured using an anemometer orultrasonic wind sensor located on the ground station.

At block 406, method 400 includes selecting a control scheme for one ormore power generation components of the aerial vehicle, based on thedetermined power-generation state. In one embodiment, two controlschemes are described; however, additional control schemes are possibleand this example should not be construed as limiting the quantity ofcontrol schemes.

As illustrated by block 408, if the aerial vehicle is in anefficiency-limited power state, a first control scheme may be selected.As illustrated by block 410, if the aerial vehicle is in atemperature-limited power state, a second control scheme may beselected. Preferably, these control schemes are different controlschemes. Alternatively, these control schemes may be the same controlschemes, but with different operational parameters.

One control scheme may comprise controlling at least one rotor viasetting an advance ratio for the rotor. Under an advance ratio controlscheme, rotors may operate according to an advance ratio that maydescribe how the blades of the rotor advance or screw into the wind(i.e., air). For example, the advance ratio at which a rotor isoperating may be the ratio between the distance the rotor moves forwardthrough the air during one revolution, and the diameter of the rotor.Mathematically, the advance ratio may be represented as J=V_(a)/n*D. Jis the non-dimensional term representing the advance ratio. V_(a) is thedistance of advance per unit time, which may be referred to as theapparent or local wind speed seen by the rotor, or as the airspeed ofair into the rotor. This is generally a significantly different valuethan the natural wind speed in which the AWT is operating; for example,70 meters-per-second is a reasonable airspeed for the rotors to seeduring crosswind flight mode, as opposed to the 3.5 to 10meters-per-second wind speed that the aerial vehicle may be operatingwithin during that time. n represents the rotational speed of the rotorin revolutions per unit time. D represents the diameter of the rotorblades. Alternatively, the advance ratio may be mathematically definedas J=πV/wR where π is the mathematical constant pi, V is equivalent toV_(a) in the previous example, w represents the angular rate of therotor (in rad/s), and R represents the radius of the rotor blades.

Advance ratio may be thought of as the effective angle-of-attack of therotor, or alternatively as the pitch angle of the helical path the tipsof the rotor traverse as they move through the air. Advantageously,advance ratio control automatically takes into account how fast theaerial vehicle is traveling. It is useful for preventing rotor bladestall and also for controlling rotors to their maximum drag state(preferably with some margin away from a stall). In some embodiments,aerial vehicles may be designed such the rotor blades are as small asthey can be while meeting optimal efficiency metrics. For example, forsome aerial vehicles, optimal efficiency is obtained when the dragcoefficient of the rotors is one-half of the drag coefficient of therest of the aerial vehicle system (i.e., C_(D(prop))=½C_(D(sys))), sooperating in the maximum drag state may be preferred when the aerialvehicle is operating in the efficiency-limited power generation state.

In an alternative embodiment, two or more rotors may be controlled bysetting an independent advance ratio for each rotor. For example, inorder to produce lift, the airspeed over the top of the wing of theaerial vehicle (and thus at the top rotors) may be higher than that atthe bottom of the wing (and thus at the bottom rotors). Also, if thewing is flying in a circle or similar pattern, the outboard rotorstravel faster than the inboard rotors. Thus, the top rotors and outboardrotors have the potential to generate much higher powers and heat thanthe bottom rotors and inboard rotors. Under this condition, the advanceratio control may be used in the temperature-limited section of thepower curve. The top and outer rotors may be set to a lower advanceratio (i.e., lower drag coefficient) and the lower and inner rotors maybe set to a higher advance ratio (i.e., higher drag coefficient). Thismay have the effect of more evenly distributing the power generationamong the generators.

Another control scheme may be thrust/drag control. Under this scheme,each rotor is commanded to produce a specified thrust or drag. To dothis, airspeed at the rotor is determined (or estimated). An angularrate of the rotor blades is then calculated that will produce therequired thrust or drag. Thrust/drag control is useful for applyingspecific turning moments to the aircraft (e.g., for hovering orturning). It is also useful for attempting to produce the optimal amountof power (e.g., at C_(D(prop))=½C_(D(sys))) during the efficiencylimited power generation state. Thrust/drag control may also be usefulfor dealing with thermal limits in the temperature-limited powergeneration state, or for other limiting states, such as when tension onthe tether may limit operation of the aerial vehicle. For example,increasing C_(D(prop)) may decrease tension in the tether, whiledecreasing C_(D(prop)) may increase cooling of the generator.

Another control scheme may be torque control. Torque in a generator (orin a motor, when the generator may be acting as a motor to drive therotor, as opposed to being driven by the rotor) is nearly directlymeasurable as it is proportional to the current that passes through thegenerator's coils. As heating of this power generation components islargely determined by Joule heating in the coils (P_(heat)=I²R), currentand/or torque limiting is a useful parameter to control undesirableheating. In the efficiency-limited section of the power curve, putting amaximum torque limit on the generators is useful for preventingtemporary or unexpected overheating. For example, this may occur whenthe generators are used as motors to help turn the aerial vehicle. Inthe temperature- or power-limited states, the generators can beconfigured to receive (and/or the motors can be configured to produce) amaximum amount of torque which corresponds to the maximum heatingallowed in the generator/motor. This torque set-point may be scaled withwind speed (or mean wing speed of the aerial vehicle) to follow thepredicted cooling behavior generator (or generator as motor).

Finally, referring now to block 412, method 400 includes operating thepower-generation components of the aerial vehicle according to theselected control scheme.

Summarily, method 400 may allow the aerial vehicle to improve powergeneration in variable wind conditions. Selection of various controlschemes disclosed herein may allow for efficient operation of an aerialvehicle, while preventing or limiting overheating or over-powerconditions.

FIG. 6 illustrates another embodiment of a method 600. At block 602,method may comprise operating an aerial vehicle of an air wind turbine(AWT) in a crosswind-flight mode to generate power. The aerial vehiclemay be coupled to a ground station through a tether. The aerial vehiclemay include at least one rotor coupled to at least one generator for thepurpose of power generation when the aerial vehicle operates in thecrosswind-flight mode. While the aerial vehicle is in thecrosswind-flight mode, the method may continue at block 604 withdetermining a power generation state of the aerial vehicle. The powergeneration state may be one of a plurality of power generation states ofthe aerial vehicle. At block 606, the method 600 may further includeselecting, based on the determined power-generation state, a controlscheme for one or more power-generation components of the aerial vehicleand operating the one or more power-generation components of the aerialvehicle according to the selected control scheme.

In another embodiment, an airborne wind turbine (AWT) system maycomprise an aerial vehicle configured to operate in a crosswind-flightmode to generate power. The aerial vehicle may be coupled to a groundstation through a tether. The aerial vehicle may include at least onerotor coupled to at least one generator for the purpose of powergeneration when the aerial vehicle operates in the crosswind-flightmode. The system may further include a control system, such as controlsystem 248. The control system may be configured to, while the aerialvehicle is in the crosswind-flight mode, determine a power generationstate of the aerial vehicle. The power generation state may be one of aplurality of power generation states of the aerial vehicle. Theplurality of power generation states may include, but are not limitedto, an efficiency-limited power generation state and atemperature-limited power generation state. Other power generationstates are possible as well. The control system may be furtherconfigured to include selecting, based on the determinedpower-generation state, a control scheme for one or morepower-generation components of the aerial vehicle. A first controlscheme may be selected if the aerial vehicle is in theefficiency-limited power generation state. A second control scheme maybe selected if the aerial vehicle is in the temperature-limited powergeneration state. Additional or other control schemes may be selected aswell, and may be based on power generation states other than anefficiency-limited power generation state and a temperature-limitedpower generation state. The control system may also be furtherconfigured to operate the one or more power-generation components of theaerial vehicle according to the selected control scheme.

IV. Conclusion

The particular arrangements shown in the Figures should not be viewed aslimiting. It should be understood that other embodiments may includemore or less of each element shown in a given Figure. Further, some ofthe illustrated elements may be combined or omitted. Yet further, anexemplary embodiment may include elements that are not illustrated inthe Figures. Additionally, while various aspects and embodiments havebeen disclosed herein, other aspects and embodiments will be apparent tothose skilled in the art. The various aspects and embodiments disclosedherein are for purposes of illustration and are not intended to belimiting, with the true scope and spirit being indicated by thefollowing claims. Other embodiments may be utilized, and other changesmay be made, without departing from the scope of the subject matterpresented herein. It will be readily understood that the aspects of thepresent disclosure, as generally described herein, and illustrated inthe figures, can be arranged, substituted, combined, separated, anddesigned in a wide variety of different configurations, all of which arecontemplated herein.

We claim:
 1. A method comprising: operating an aerial vehicle of an airwind turbine (AWT) in a crosswind-flight mode to generate power, whereinthe aerial vehicle is coupled to a ground station through a tether,wherein the aerial vehicle includes at least one rotor coupled to atleast one generator for power generation when the aerial vehicleoperates in the crosswind-flight mode; and while the aerial vehicle isin the crosswind-flight mode: determining, based on sensor data, anamount of power generated by the aerial vehicle; comparing the amount ofpower generated by the aerial vehicle to both a first threshold powerand a second threshold power; if the comparison indicates that theamount of power generated by the aerial vehicle is less than both thefirst threshold power and the second threshold power, then determiningthat the aerial vehicle is in a first power generation state, andoperating one or more power-generation components of the aerial vehicleaccording to a first control scheme, wherein the first control schemeincludes setting a first drag coefficient of the at least one rotor; ifthe comparison indicates that the amount of power generated by theaerial vehicle is greater than or equal to the first threshold power andless than or equal to the second threshold power, then determining thatthe aerial vehicle is in a second power generation state, and operatingthe one or more power-generation components of the aerial vehicleaccording to a second control scheme, wherein the second control schemeincludes decreasing the setting of the first drag coefficient of the atleast one rotor to a second drag coefficient to control heat generationof the at least one rotor; and if the comparison indicates that theamount of power generated by the aerial vehicle is greater than both thefirst threshold power and the second threshold power, then determiningthat the aerial vehicle is in a third power generation state, andoperating the one or more power-generation components of the aerialvehicle according to a third control scheme, wherein the third controlscheme includes setting a torque limit for the at least one rotor suchthat the at least one generator of the aerial vehicle operates at arated power threshold.
 2. The method of claim 1, wherein each of thefirst control scheme, the second control scheme, and the third controlscheme is selected from a plurality of control schemes comprising atleast the first control scheme, the second control scheme, and the thirdcontrol scheme.
 3. The method of claim 1, wherein the first controlscheme maximizes power generation, wherein the second control schemecontrols heat generation associated with power generation, and whereinthe third control scheme controls both power generation and heatgeneration associated with power generation.
 4. The method of claim 1,wherein the first control scheme is selected, and wherein operating theone or more power-generation components of the aerial vehicle accordingto the first control scheme comprises controlling the at least one rotorvia setting an advance ratio for the at least one rotor, wherein the AWTis operating below a rated power of the AWT, and wherein setting theadvance ratio for the at least one rotor comprises setting a fixedadvance ratio for the at least one rotor that does not equal or exceedan advance ratio resulting in rotor stall.
 5. The method of claim 1,wherein the second control scheme is selected, and wherein the at leastone rotor coupled to the at least one generator comprises a first rotorcoupled to a first generator and a second rotor coupled to a secondgenerator, and wherein the first rotor is subject to a first airspeedand the second rotor is subject to a second airspeed that is greaterthan the first airspeed, and wherein operating the one or morepower-generation components of the aerial vehicle according to thesecond control scheme comprises: operating the first rotor at a firstadvance ratio; and operating the second rotor at a second advance ratiothat is less than the first advance ratio such that power generated bythe second generator is substantially equivalent to power generated bythe first generator.
 6. The method of claim 1, wherein the third controlscheme is selected, and wherein operating the one or morepower-generation components of the aerial vehicle according to the thirdcontrol scheme further comprises: determining a maximum current that maysafely pass through the at least one generator; for the at least onerotor coupled to the at least one generator, determining a maximum rotortorque that corresponds to the maximum current; and setting the torquelimit of the at least one rotor to the maximum rotor torque.
 7. Themethod of claim 6, wherein the at least one generator operates as amotor supplying torque to the at least one rotor.
 8. The method of claim1, wherein the first threshold power corresponds to a point on apower-generation curve where an incremental increase in power generationper unit of wind speed drops due to an effect of heating in thepower-generation components.
 9. An airborne wind turbine (AWT) systemcomprising: an aerial vehicle configured to operate in acrosswind-flight mode to generate power, wherein the aerial vehicle iscoupled to a ground station through a tether, and wherein the aerialvehicle includes at least one rotor coupled to at least one generatorfor power generation when the aerial vehicle operates in thecrosswind-flight mode; and a control system configured to: (i) while theaerial vehicle is in the crosswind-flight mode, receive sensor data todetermine an amount of power generated by the aerial vehicle; (ii)compare the amount of power generated by the aerial vehicle to both afirst threshold power and a second threshold power; (iii) if thecomparison indicates that the amount of power generated by the aerialvehicle is less than both the first threshold power and the secondthreshold power, then determine that the aerial vehicle is in a firstpower generation state, and operate one or more power-generationcomponents of the aerial vehicle according to a first control scheme,wherein the first control scheme includes setting a first dragcoefficient of the at least one rotor; (iv) if the comparison indicatesthat the amount of power generated by the aerial vehicle is greater thanor equal to the first threshold power and less than or equal to thesecond threshold power, then determine that the aerial vehicle is in asecond power generation state, and operate the one or morepower-generation components of the aerial vehicle according to a secondcontrol scheme, wherein the second control scheme includes decreasingthe setting of the first drag coefficient of the at least one rotor to asecond drag coefficient to control heat generation of the at least onerotor; and (v) if the comparison indicates that the amount of powergenerated by the aerial vehicle is greater than both the first thresholdpower and the second threshold power, then determine that the aerialvehicle is in a third power generation state, and operate the one ormore power-generation components of the aerial vehicle according to athird control scheme, wherein the third control scheme includes settinga torque limit for the at least one rotor such that the at least onegenerator of the aerial vehicle operates at a rated power threshold. 10.The system of claim 9, wherein the first control scheme maximizes powergeneration, wherein the second control scheme controls heat generationassociated with power generation, and wherein the third control schemecontrols both power generation and heat generation associated with powergeneration.
 11. The system of claim 9, wherein the first control schemeis selected, and wherein operating the one or more power-generationcomponents of the aerial vehicle according to the first control schemecomprises controlling the at least one rotor via setting an advanceratio for the at least one rotor, and wherein setting the advance ratiofor the at least one rotor comprises setting a fixed advance ratio forthe at least one rotor that does not equal or exceed an advance ratioresulting in rotor stall.
 12. The system of claim 9, wherein the secondcontrol scheme is selected, and wherein the at least one rotor coupledto the at least one generator comprises a first rotor coupled to a firstgenerator and a second rotor coupled to a second generator, and whereinthe first rotor is subject to a first airspeed and the second rotor issubject to a second airspeed that is greater than the first airspeed,and wherein, to operate the one or more power-generation components ofthe aerial vehicle according to the second control scheme, the controlsystem is further configured to: operate the first rotor at a firstadvance ratio; and operate the second rotor at a second advance ratiothat is less than the first advance ratio such that power generated bythe second generator is substantially equivalent to power generated bythe first generator.
 13. The system of claim 9, wherein the thirdcontrol scheme is selected, and wherein, to operate the one or morepower-generation components of the aerial vehicle according to the thirdcontrol scheme, the control system is further configured to: determine amaximum current that may safely pass through the at least one generator;for the at least one rotor coupled to the at least one generator,determine a maximum rotor torque that corresponds to the maximumcurrent; and set the torque limit of the at least one rotor to themaximum rotor torque.
 14. The system of claim 13, wherein the at leastone generator operates as a motor supplying torque to the at least onerotor.
 15. The system of claim 9, wherein the first threshold powercorresponds to a point on a power-generation curve where an incrementalincrease in power generation per unit of wind speed drops due to aneffect of heating in the power-generation components.